Products such as crude oil, refined petroleum, natural gas, etc., are frequently transported cross-country by pipelines. These pipelines are usually long and have a fairly large diameter. For example, the diameter may be in a range from somewhat less that 20 inches on up to 50 inches or more. In order to increase the rate of flow through the pipelines, it is customary to employ high pressures. To prevent a catastrophic failure, it is essential the pipeline be free of any defects which might result in a weak spot.
A pipeline of this nature is normally built by distributing a series of individual sections of pipe along the pipeline right-of-way. The adjacent ends of the succeeding sections are then welded together to form a continuous pipeline. Following this the finished pipeline is normally buried. It will thus be seen that although the pipeline is continuous and unbroken it has a series of circumferential or girth welds which are spaced at intervals corresponding to the lengths of the original sections. Even though these welds are made in the field under less than ideal conditions, it is essential each one be of a high quality and free from defects such as cracks, inclusions, etc., which might reduce the strength thereof.
At the present time the most reliable means of inspecting welds of this type is to utilize X-rays. The most satisfactory way of X-raying a girth weld is to wrap a belt containing a strip of photographic film around the outside of the pipe so as to cover the weld. The entire weld (i.e., 360.degree.) is then irradiated from the center of the pipe with X-rays. The X-rays penetrate through the weld and expose the film in the belt on the outside of the pipe.
One means of accomplishing this is to utilize an X-ray machine or pipeline crawler such as dislcosed and claimed in U.S. Pat. No. 2,532,536 entitled "Method and Apparatus for Locating Welds in Hollow Metal Bodies;" U.S. Pat. No. 2,547,996 entitled "Portable X-Ray Unit;" and U.S. Patent No. 2,604,521 entitled "Conduit Tractor," all of the aboveidentified patents were filed in the name of Cormack E. Boucher and are assigned of record to Automation Industries, Inc.
A pipeline crawler of this nature travels inside the pipeline, stops at the location of a girth weld, irradiates the weld with X-rays to expose the film and then moves onto the next weld where it again stops and repeats the X-ray operation.
Pipeline crawlers of this nature have been very successful in X-raying the girth welds. However, they have required a remote power source and some form of remote control. As a result they utilized an electrical cable or umbilical cord which they drag through the pipeline. An operator stationed at the end of the pipeline has then controlled the crawler by sending signals over the cable. This has several disadvantages.
It requires an operator with the resultant expenses, etc. In addition, since the operator is located at the end of the pipe, difficulties sometimes occur in locating the weld and stopping the crawler at precisely the location of the weld. Moreover, there are practical limits as to how far the crawler can travel into the pipeline while dragging the cable.